1. Field of the Invention
The present invention relates to an arithmetic unit for image transformation and a monitoring system. More particularly, the invention relates to an arithmetic unit for image transformation for transforming an image obtained by using a fisheye lens into a plane image for display and a monitoring system having the arithmetic unit.
2. Description of the Related Art
An arithmetic unit for image transformation is used in, for example, a monitoring system using a monitoring camera. The operator monitors the state in a space (for example, shop) in which the monitoring camera is installed, by watching images from the monitoring camera displayed on a monitor provided for the monitoring system. When a lens attached to the camera is an ordinary standard lens, the space can be monitored only within the range of the angle of view of the standard lens. In order to monitor the entire space in which the camera is installed, it is necessary to provide a mechanism for properly changing the orientation of the camera. In case of providing such a mechanism, the cost increases and the camera has to be remote controlled. It consequently becomes hard for the operator to handle it.
There is an idea such that a fisheye lens having the wide angle of view is attached to the camera and monitoring is performed by using the fisheye lens. An image produced by the fisheye lens is, however, distorted as compared with an image obtained by using the standard lens and is very hard for the operator to watch it. A technique for transforming an image produced by the fisheye lens into a plane image is disclosed as a camera orientation system in WO92/21208.
The system transforms a circular image obtained by using the fisheye lens into an image produced by a normal image pickup lens (for example, standard lens) by an arithmetic process and a plane image seen from an arbitrary view point can be obtained. In this case, when a high speed arithmetic unit is used, plane image data can be obtained at real time rates only by software. In most of the cases, it takes long time for a communicating process, operation as a human interface, and the like in a terminal device connected to a network. It is therefore preferable to realize a part in which the same process is repeated in an image transforming process, by hardware.
Since the projecting method (stereoscopic projection, equidistant projection, orthogonal projection, or the like) of projecting a fisheye lens image onto an image pickup device (such as CCD) provided for the camera is determined at the time of designing, by executing arithmetic computations according to mathematical expressions by hardware, the image can be transformed to a plane image.
As disclosed in WO92/21208, however, when calculations upon transformation are expressed by mathematical equations and executed according to the mathematical equations by hardware, it is necessary to realize many calculations of not only addition, subtraction, multiplication, and division, but also square root, trigonometric function, and the like by the hardware. The unit consequently cannot help becoming expensive inevitably.
The invention has been achieved in consideration of the actual condition of the conventional technique and it is an object of the invention to provide an arithmetic unit for image transformation capable of providing inexpensive hardware for transforming a fisheye image obtained by using a fisheye lens into a plane image for display.
In order to achieve the object, an arithmetic unit for image transformation according to the invention, for transforming a fisheye image obtained by using a fisheye lens into a plane image for display, comprises:
a first coordinate calculating unit for obtaining first projection coordinates on a fisheye image face as an imaginary object face derived by projecting coordinates on the plane image; and
a second coordinate calculating unit for obtaining second projection coordinates derived by projecting the first projection coordinates obtained by the first coordinate calculating unit onto the fisheye image face.
The action of the configuration is as follows:
Step 1. The first projection coordinates on the fisheye image face as an imaginary object face obtained by projecting coordinates on the plane image are derived by the first coordinate calculating unit; and
Step 2. The second projection coordinates obtained by projecting the first coordinates onto the fisheye image face are derived.
That is, when an image is transformed, the coordinate calculations are not performed by mathematical equations at once but are executed by stages. Consequently, the calculating unit can be constructed by combining simple arithmetic circuits and the hardware part in the arithmetic unit for image transformation can be realized at low cost.
As a preferred embodiment of the invention, a logic circuit for arithmetic operation has a pipelined architecture in each of the first and second coordinate calculating units.
By making the logic circuit have the pipelined architecture, the coordinate transformation can be sequentially performed by stages and the circuit configuration can be partially simplified. As a result, the hardware part in the arithmetic unit for image transformation can be realized at low cost.
As another preferred embodiment of the invention, the logic circuit for arithmetic operation is limited to calculations of addition, subtraction, multiplication, and square root, and division and other function calculations are handled by referring to a lookup table.
In case of executing the calculations according to the equations by the hardware, it is necessary to realize many arithmetic operations such as addition, subtraction, multiplication, and division, and in addition, square root, trigonometric function, and the like by hardware. It is consequently necessary to use a large-scale circuit part. Especially, depending on the projecting method of the fisheye lens, there may be a case such that approximation occurs by trigonometric function or infinite polynomial. The logic circuit for arithmetic operation is consequently limited to addition, subtraction, multiplication, and square root. With respect to a part expressed by division or other function, a lookup table is referred to. Consequently, the hardware can be simplified and it can contribute to reduce the cost of the arithmetic unit for image transformation.
According to further another preferred embodiment of the invention, the second coordinate calculating unit is used to obtain the second projection coordinates by multiplying the first projection coordinates by a predetermined coefficient and the predetermined coefficient is obtained from the lookup table.
When the projecting method of the fisheye lens differs, the coefficient becomes different. Only by changing data in the lookup table, the invention can deal with a different projecting method and it is unnecessary to replace the hardware part. The cost of the hardware part in the arithmetic unit for image transformation can be therefore reduced also with respect to the point.
A monitoring system according to the invention is characterized by comprising the arithmetic unit for image transformation. According to the arithmetic unit for image transformation, the arithmetic unit can be constructed by combining simple arithmetic circuits and the cost of the hardware part in the arithmetic unit for image transformation can be reduced, so that an inexpensive monitoring system can be provided.